1. Field of the Invention
This invention relates to signal receivers, especially receivers for radio signals as are used in base transceiver stations (BTSs) and mobile phones or mobile communication systems.
FIG. 1 shows the architecture of a receiver suitable for receiving GMSK (Gaussian minimum shift-keyed) modulated signals. Such signals generally comprise a train of symbols equally spaced in time. Before the modulation each symbol takes one of two values, conventionally denoted +1 and −1. After GMSK modulation the signals comprise of complex valued samples each having a I (in-phase) and a Q (quadrature-phase) component. These signals are received at an antenna 1 and pre-processed by a front-end section shown schematically at 2. The pre-processing involves amplification and basic filtering. The signal is then sampled at the symbol frequency by a sampling unit 3 to form a train of complex samples denoted r. The samples are then demodulated by a demodulation unit 4 to form a train of symbol estimates y. The symbol estimates are passed to an equaliser 5, which forms an estimate of the received symbols and outputs them at 6 for further processing to decode the information represented by them. The sampling unit in this receiver provides only one sample per symbol. Further processing of the signal happens by considering one sample per symbol. Therefore, it may be called a “symbol-spaced” receiver.
2. Description of the Related Art
During their passage from the transmitter to the receiver the symbols will have been subject to interference. The interference may include:
Interference from other signals on the channel. That may be spectrally neutral (“white”) interference; or interference that is spectrally biased (“coloured” interference). Coloured interference often results from interference from other signals of the same communication system.
Interference arising from the propagation of the signal in question itself. This includes inter-symbol interference: interference between nearby symbols in that signal due to their having travelled from the transmitter to the receiver on multiple paths of different lengths.
In order to allow the signal to be recovered reliably at the receiver, the effects of interference must be taken into account. To achieve this the symbol estimates y are passed to a channel estimator 7 which forms an estimate of the effects of the channel by analysing the form of a known training sequence of symbols as received. The channel estimator forms an estimate h of the signal prior to distortion by the channel. h and y are passed to an SNR estimator 8 which estimates the signal-to-noise ratio (SNR) of the received signal. h is also passed to an auto-correlation computation unit 9 which generates a reference signal F. F and the SNR estimate are passed to a GMSK scaling unit 10 which scales h and F by the SNR estimate and produces corresponding h′ and F′. h′ represents a set of filter taps gauged to represent the characteristics of the channel. Those are applied to a matched filter unit 11 of the equaliser 5 which filters the samples y in accordance with the taps h′ to form Z. Z and F′ are passed to an equaliser 12 which estimates the symbols that made up the received signal. One way in which that may be done is by computing based on the channel estimate h′ the likely effect of the channel on all possible sets of received symbols, and comparing those sets with the actual received signal to determine the best fit. The symbols in the best-fitting set are adopted as the estimate that is output at 6 for further processing.
One approach that has been taken to improve the accuracy of decoding is to oversample the received signal. By taking samples of the received signal at a higher frequency than the symbol rate more data on the received signal is obtained, which can be used to improve the decoding process. Since the additional samples have been generated from actual sampling of the received signal they include additional data over that contained in a set of samples taken at the symbol rate, which can be used (e.g. by means of a temporal whitening technique) to help eliminate co-channel and adjacent-channel interference. This type of receiver may be called a “fractionally-spaced” receiver. However, in some situations the receiver may not have the capability of performing oversampling, and so this route is not available.
One example of this situation is where a radio-frequency module (RFM) of receiver has been designed in such a way that it provides only one sample per symbol to the base-band module where the rest of the signal processing takes place. Radio-frequency module is the receiver front-end component that performs amplification, front-end filtering and sampling. Usually this component comes as a hardware solution. The base-band module is where digital signal processing takes place. Usually this part is implemented partly through software and partly through hardware.
In recent days the radio-frequency module of a receiver has been designed to provide more than one sample per symbol to the base-band unit. At the same time, the digital signal processing functions in the base-band unit of receiver make use of this extra group of samples to enhance the performance of receiver. But such RFMs introduce additional cost to the overall receiver.
There are customers who do not wish to replace their old radio-frequency modules, but want to change their base-band units. They may not want the best but at least improved performance from their receiver by replacing only the base-band modules. Therefore, it is desired to enhance the old symbol-spaced GMSK receiver and integrate it into the new base-band unit so that it can cope with one sample per symbol, yet with better performance.
As mentioned above, in a typical receiver the signal processing in the base-band unit is split between software and dedicated hardware elements. The software is typically executed by a DSP (digital signal processor) which can only carry out a limited number of operations in the time available between successive symbols. Therefore, it is essential to streamline the software so that it can be carried out by suitable DSPs. The hardware may be pre-existing, or may be shared between different platforms, so it is desirable if the receiver design can take advantage of standard hardware.